netinet6: use in6_ifmtu() instead of IN6_LINKMTU() macro

There should be no functional change. If there are any performance
concerns with a function call, with the future changes, that would move
ND6 bits into in6_ifextra, this function would be easily inline-able.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D54724